1. Field of Invention
The invention relates to an ion sensing circuit, and more particular to a circuit adopting Ion-Sensitive Field Effect Transistors that is compatible with CMOS technology and implemented by integrated circuits.
2. Related Art
Silicon-based semiconductor micro sensors are now able to react to the ion concentration (activity). The Ion-Sensitive Field Effect Transistor (ISFET), which is a micro sensing element combining electrochemistry with microelectronics technology, was introduced in the 1970s. The ISFET selectively senses the ion concentration in an electrolyte. The ISFET is a trans-resistance element, which has the features of low output impedance of MOSFET and operation of Ion Selective Electrode (ISE). The ISFET has features of rapid reaction time, high sensitivity, batch processing, small size and single chip integration. Furthermore, it can be implemented by CMOS technology. These advantages make it the first choice for VLSI electrochemistry sensing array.
Compared with MOSFET, ISFET replaces the metal or polysilicon gate with electrolytes and reference electrode. Different concentration of electrolyte components causes corresponding variations of the threshold voltage of the ISFET. Through the reaction of the sensing membrane and the electrolyte, the concentration of H+ or other ions can be acquired by sensing circuits.
Many sensing circuits based on the above concept have been proposed in the prior art. One of which is shown in FIG. 1. The sensing circuit of FIG. 1 detects the ion concentration of the solution with features of constant voltage/constant current operation mode, and floating reference electrode. The drain terminal of the transistor ISFET is connected to the output terminal of the first amplifier OP1, where a constant voltage, e.g., 0.7 volts in the figure, is fed to its positive terminal. The negative terminal is connected to the output terminal. The source terminal of the transistor ISFET is coupled with the negative terminal of the second amplifier OP2, and coupled with the ground via a resistor R. A constant voltage, e.g., 0.5 volts in the figure, is fed to the positive terminal of the second amplifier OP2. The output terminal of the second amplifier OP2 is coupled with the reference electrode Ref of the transistor ISFET. With this configuration, two constant voltages input to the two positive terminals of the amplifiers cause the source terminal S and the drain terminal D of the transistor ISFET to keep a constant drain-source voltage difference. The solution of the ion concentration creates the connection between the reference electrode Ref and gate sensing membrane (terminal G). The potential difference between the gate sensing membrane and the reference electrode Ref is determined by the ion concentration of the solution.
The sensing circuit in FIG. 1 is easy to be implemented by integrated circuits. The measured signal is the output from the amplifier OP2 connected with the reference electrode Ref.
However, once the sensing circuit in FIG. 1 is applied to multiple sensors or sensor arrays, since one reference electrode is necessary for each transistor ISFET, the increasing number of sensors leads to increasing number of reference electrodes. This is not economically viable, and is not suitable for mass production or commercial development.
The circuit in FIG. 2 adopts one sensing circuit for all the sensors, i.e., only one reference electrode is used to detect the ion concentration. The circuit in FIG. 2 is composed of a first transistor ISFET1 and a second transistor ISFET2. The drain terminals of the two transistors are connected together. The drain terminal of the transistor ISFET1 is coupled with the output terminal of the first amplifier OP1, where a constant voltage, e.g., 0.7 volts in the figure, is fed to its positive terminal. The negative terminal of the first amplifier OP1 is connected to the output terminal. The source terminals of the two transistors coupled with analog switches CH1 and CH2 respectively, which are switched by a multiplexer. The switches CH1 and CH2 are also coupled with the negative terminal of the second terminal OP2. The negative terminal is also coupled with the ground via a resistor R. A constant voltage, e.g., 0.5 volts in the figure, is fed to the positive terminal of the second amplifier OP2, whose output terminal is coupled to the reference terminal. The two constant voltages input to the two positive terminals of the amplifiers cause the source terminal and the drain terminal of the transistor ISFET to keep a constant voltage difference.
The main advantages of the circuit in FIG. 2 are reduced chip size and power consumption. However, the problems of the conductivity of the switches, the rising temperature upon switching and noise interference need to be solved. Furthermore, only one ISFET operates at one time. Sufficient stable time is needed before switching to the next ISFET so all the sensors cannot operate at the same time. The situation is more serious when the number of ISFETs in the sensor array increases. The detected signal implies the drift error of the ISFET owing to the switching time for completing the detection by ISFETs.
The multiple sensing circuit presented in FIG. 2 has essential drawbacks. The supplying voltage is multiplexed by CH1 and CH2 from ISFET1 to ISFET2 and both ISFETs cannot be supplied simultaneously. This may cause instability of the output signal Vout due to transient processes resulting from the switching. Another drawback is increasing of measuring time resulting from the serial mode of operation and the above-mentioned transient processes.
Therefore, for overcoming of the above-mentioned drawbacks, another circuit is proposed in FIG. 3. The configuration of this circuit involves grounding the reference electrode and a bridge-type circuit with a floating voltage source.
The bridge-type configuration includes a current source Iref, a constant voltage source that is generated together with a variable resistor Rv, and an operational amplifier OP. The Zener diode ZN1 provides a specific reference voltage. The operational amplifier OP, the resistor Ra, Rb, and Rc, and the ISFET form an electrical bridge network. The advantage of this configuration is grounding the reference electrode Ref, so that the only one reference electrode is necessary for multiple ISFET detection. The circuit has a wide range of operations and is suitable for ISFETs with unspecified characteristics. However, the Zener diode ZN1 needs a special manufacture technology, and the voltages of its two sides are floating. The circuit in FIG. 3 is not suitably implemented by a standard CMOS technology.